Input protection devices commonly found on antennas are configured to prevent unwanted transient signals, such as current spikes, from propagating through the supporting circuitry attached to the output of the antenna. These transient signal spikes may be caused by various phenomena including static electricity, noise, or may result from the presence of a large transmitted signal. However, to attenuate these transients, various limiting systems have been utilized, but suffer from inadequate performance or create unwanted characteristics that impede the antenna's overall performance.
One such limiting system utilizes a pair of parallel, oppositely connected, signal diodes as a limiting circuit to achieve input protection for the remaining portion of an antenna's supporting circuitry. Because the diodes conduct some amount of current between their on and off states, the system provides “soft” limiting. Furthermore, when the diodes conduct signals that are below the threshold values established by the diodes, unwanted distortion of the received signal generally results. Additionally, if multiple signals are detected by the antenna, intermodulation distortion, or IMD, may occur due to the design of the limiting network. Furthermore, the low voltage threshold of the diodes results in the possibility that a large out-of-band signal may drive the diodes into their limiting region. As a result, IMD may be induced in the desired in-band signals being received by the antenna. To overcome the low voltage threshold, inherent with the parallel diode limiting circuit, several diodes are typically arranged in series in each parallel leg of the limiting circuit, thereby increasing the overall threshold voltage of the limiting system. However, while the threshold voltage of the system is increased, thereby reducing the amount of distortion that may be induced into the system, the limiting characteristics of the system are further degraded or “softened.” As such, there is a trade-off between having high limiting thresholds, and “hard” limiting, with the selection of one characteristic resulting in the degradation of the other.
Another commonly used limiting circuit which provides input protection for active antennas utilizes series connected zener diodes, with their anodes coupled together, or with their cathodes coupled together. Because zener diodes, when reverse biased, have a threshold voltage much greater than conventional diodes, the limiting network overcomes the problems of low threshold voltage associated with the previously discussed system. However, this limiting network reduces the performance of the antenna to which the limiting network is coupled due to the zener's high capacitance, and slow switching action between the forward biased zener diode's on and off states. Additionally, the capacitance of the zener diode increases greatly in a non-linear manner as the applied voltage from the signal increases, resulting in IMD and other distortion forms being introduced into the antenna network. Also because of the slow switching action between the on and off states of the forward biased zener diode, distortion of a received signal can also result after a transient signal has decayed after being limited by the system.
Phasing networks or circuits used in conjunction with a phased array antenna system are used to establish a directional receiving pattern. Of primary importance are the location of nulls in the antenna's receiving pattern, and the orientation of the null in a desired direction. As a result, the phase array antenna is insensitive to signals arriving at the antenna in the direction of the null, while the remaining portion of the antenna's receiving pattern continues to be sensitive to transmitted signals. To achieve the delay necessary to create a directional receiving pattern, one or more delay lines, each typically comprising a specific length of coaxial cable of a length corresponding to a desired phase delay are used. However, because these delay lines are frequency dependant, the amount of phase delay provided changes as the signal frequency detected by the antenna array is altered. As such, when it is desired that the antenna system be responsive to signals of a new frequency, the delay provided by the delay lines changes. Thus, the null created in a first direction according to the delay provided by the original delay line is altered because the amount of phase delay provided by the delay line has changed due to the new operating frequency of the antenna system.
Therefore, there is a need for a limiting circuit for use with an active antenna that provides sharp limiting action to received signals, while having a high limiting threshold characteristic. Additionally, there is a need for a limiting circuit that can be formed from a small number of components to provide reduced cost and manufacture. Furthermore, there is a need for an isolation circuit that provides isolation between the antenna element and any cable, such as an element transmission line, coupled to the output of an antenna unit. There is yet a further need for a phasing circuit for a phased array antenna system that is frequency independent, and that maintains a null in a desired direction over a wide bandwidth of received signals.